1. Field of the Invention
The present invention relates to a radiation image data obtaining method and a radiation image data obtaining apparatus for obtaining an image signal representing radiation image information from a solid-state radiation detector, especially from a detector having a stripe electrode, and also to the radiation detector used therefor.
2. Description of the Related Art
In today's radiation photography aimed at medical diagnoses or the like, radiation image reading and recording apparatuses using solid-state radiation detectors (whose main part comprises semiconductors) for outputting image signals representing radiation image information by detecting radiation have been known. As the solid-state radiation detectors used therefor, various types have been proposed and put into practice.
In terms of an electric charge generation process in which radiation is converted into an electric charge, solid-state radiation detectors of different types, such as optical conversion type detectors (see Japanese Unexamined Patent Publication Nos. 59(1984)-211263 and 2(1990)-164067, PCT International Publication No. W092/06501, and SPIE Vol. 1443 Medical Imaging V; Image Physics (1991), p. 108-119, for example) and direct conversion type detectors (MATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTORS, Lawrence Berkeley Laboratory. University of California, Berkeley, Calif. 94720 Xerox Parc. Palo Alto. Calif. 94304, Metal/Amorphous Silicon Multilayer Radiation Detectors, IEEE TRANSACTIONS ON NUCLEAR SCIENCE. VOL. 36. NO.2. Apr. 1989, and Japanese Unexamined Patent Publication No. 1(1989)-216290, for example) have been known. In an optical conversion type detector, light emitted from phosphor by exposing the phosphor to radiation is detected by a photoelectric conversion device and a signal electric charge thereby obtained is stored in a capacitor of the device. The stored electric charge is then converted into an image signal (electric signal) and the signal is output. In a direct conversion type detector, a signal electric charge generated within a radiation conductive material by exposing the material to radiation is collected by an electric charge collecting electrode and stored in a capacitor. The stored electric charge is then converted into an electric signal and the signal is output.
With respect to an electric charge reading process in which a stored electric charge is read out, solid-state radiation detectors of other types, such as TFT reading type detectors which read the charge by scanning TFT's (Thin Film Transistors) connected to capacitors and optical reading type detectors in which a charge is read by irradiating reading light (an electromagnetic wave for reading) thereon have been known.
The present applicant has proposed radiation detectors of improved direct conversion type (see Japanese Patent Application Nos. 10(1998)-232824 and 10(1998)-271374). A radiation detector of improved direct conversion type means a radiation detector employing both the direct conversion method and the optical reading method. The radiation detector of improved direct conversion type comprises a first conductive layer which is transparent to radiation for recording, a photoconductive layer for recording exhibiting photoconductivity (or, more accurately, radiation conductivity) when receiving the radiation for recording which has passed through the first conductive layer, an electric charge transport layer which acts approximately as an insulator to an electric charge having the same polarity as an electric charge charged in the first conductive layer while acting approximately as a conductor to an electric charge having the reverse polarity, a photoconductive layer for reading exhibiting photoconductivity (or, more accurately, electromagnetic wave conductivity) when receiving an electromagnetic wave for reading, and a second conductive layer which is transparent to the electromagnetic wave for reading, with these layers being stacked in this order. A signal electric charge (latent image electric charge) representing image information is stored at the interface (capacitor) between the photoconductive layer for recording and the electric charge transport layer. The first and the second conductive layers function as electrodes. The photoconductive layer for recording, the electric charge transport layer, and the photoconductive layer for reading comprise the main part of the solid-state detector of this type.
In the optical reading method including the improved direct conversion method, 3 types of signal detection methods are used for reading a signal electric charge stored in a capacitor. In one of the 3 methods, the second conductive layer (hereinafter called reading electrode) onto which the reading light is irradiated has a flat shape, and the signal electric charge is detected by scanning the reading electrode with spot-like reading light such as a laser beam. In the other 2 methods, the reading electrode is a stripe electrode having linear electrodes aligned in a comb-like shape. Letting the direction almost orthogonal to the longitudinal direction of the stripe electrode, that is, the direction orthogonal to the longitudinal direction of each linear electrode, be the main scan direction, while letting the longitudinal direction be the vertical scan direction, spot-like reading light scans along the main and the vertical scan directions to detect the signal charge. Alternatively, a linear light source elongated along the main scan direction scans the stripe electrode longitudinally (that is, along the vertical scan direction) to detect the signal charge.
However, in a detector employing the optical reading method, not only is a current in accordance with the electric charge accumulated in the capacitor corresponding to an area onto which the reading light irradiated, but also a dark current accumulated in the vicinity of the capacitor corresponding to the area, or in the entire capacitor, flows when the electric charge is read from the capacitor. Furthermore, in a detector having the stripe electrode as the reading electrode, a dark current flows into the linear electrode from a pixel other than a target pixel when the reading light is irradiated onto the target pixel in the longitudinal direction of the linear electrode. In other words, in the optical reading type detectors including the detectors having the stripe electrodes, an image signal output therefrom includes a dark current component in addition to a desired image signal component. Therefore, offset due to the dark current is generated in the image reproduced based on the output image signal and causes density of the image to increase. In the case of detectors having the stripe electrodes, this problem is prominent, since the dark current component along the entire longitudinal direction is added to the signal of a target pixel.